To a rotatable underwater housing of an outboard drive for boats, torque is transmitted from the motor coupled to the outboard drive via a bevel gear mounted on the inner side of the boat's hull, so that the underwater housing, under load, strives to rotate in the same direction as the rotational direction of the vertical drive shaft. Outboard drives of the type described above are used, in particular, in boats from 40 feet and upward, which have high-powered motors with high torque, for example from about 600 Nm and above. This means that the underwater housing is constantly subjected, while the boat is in motion, to a relatively high steering torque that somehow has to be balanced.
A simple and known method is, of course, quite simply to dimension the steering machinery of the drive large enough so that it is able to absorb the forces to which the motor torque gives rise, together with the steering forces when the boat yaws. This means, however, that the steering machinery has to be more generously proportioned than would be required if it merely needed to exert the force to rotate the underwater housing during yawing. Another way is to dimension the underwater housing with a very large surface area behind the steering shaft of the underwater housing. Finally, the underwater housing can be configured with an asymmetrical profile, for example with a curved aft part, which is also known.
To provide the drive with steering machinery that is more powerful than that required for the actual steering is an expensive solution. This also applies to an underwater housing having a large surface area.
Moreover, such an underwater housing increases the resistance in the water and is additionally given an unnecessary amount of weight. Also, a drive having a curved aft edge on the wing-shaped underwater housing is more difficult to produce than a symmetrical drive and the dimensional control is not as good.